test: fix unstable TestPessimisticLockOnPartition (pingcap#18963)

Co-authored-by: ti-srebot <66930949+ti-srebot@users.noreply.github.com>

session/session_test.go

@@ -3167,6 +3167,7 @@ func (s *testSessionSuite3) TestPessimisticLockOnPartition(c *C) {
 		tk1.MustExec("update forupdate_on_partition set first_name='sw' where age=25")
 		ch <- 0
 		tk1.MustExec("commit")
+		ch <- 0
 	}()
 
 	// Leave 50ms for tk1 to run, tk1 should be blocked at the update operation.
@@ -3177,6 +3178,7 @@ func (s *testSessionSuite3) TestPessimisticLockOnPartition(c *C) {
 	// tk1 should be blocked until tk commit, check the order.
 	c.Assert(<-ch, Equals, int32(1))
 	c.Assert(<-ch, Equals, int32(0))
+	<-ch // wait for goroutine to quit.
 
 	// Once again...
 	// This time, test for the update-update conflict.
@@ -3188,6 +3190,7 @@ func (s *testSessionSuite3) TestPessimisticLockOnPartition(c *C) {
 		tk1.MustExec("update forupdate_on_partition set first_name = 'xxx' where age=25")
 		ch <- 0
 		tk1.MustExec("commit")
+		ch <- 0
 	}()
 
 	// Leave 50ms for tk1 to run, tk1 should be blocked at the update operation.
@@ -3198,6 +3201,7 @@ func (s *testSessionSuite3) TestPessimisticLockOnPartition(c *C) {
 	// tk1 should be blocked until tk commit, check the order.
 	c.Assert(<-ch, Equals, int32(1))
 	c.Assert(<-ch, Equals, int32(0))
+	<-ch // wait for goroutine to quit.
 }
 
 func (s *testSchemaSuite) TestTxnSize(c *C) {